1. Field of the Invention
The present invention relates to a static random access memory (hereinafter referred to as an SRAM) and, in particular, to driver transistors in a memory cell.
2. Description of the Prior Art
In general, an SRAM memory cell comprises two transfer transistors and driver transistors constituting a flip-flop. Here, the structure of the driver transistors in particular constituting a memory area will be explained below. In FIG. 18, a gate oxide film 102 and gate electrode 103 composed of polysilicon are formed at a P well 101 in a semiconductor substrate, not shown, and then an oxide film 104 is formed over the surface of a resultant structure. A first N type region (N type impurity: phosphorus) 105 and second N type region (N type impurity: arsenic) 106 are formed in the P well 101, the surface of the resultant semiconductor structure is covered with a passivation film 107.
In order to achieve a large-capacity SRAM, it is required that those transistors involved be formed in fine form. The formation of fine transistors leads to a decrease in capacitance of the transistors and hence there is a decrease in an amount of signal charge held in the driver transistor. As a result, through a funneling phenomenon caused by an a ray, etc., a soft error may occur whereby data stored in the N type regions is destroyed. Here, the capacitance of the transistor is intended to mean a resultant capacitance (hereinafter referred to as an effective capacitance) of a capacitance of the gate oxide film 102 between the gate electrode 103 and the P well 101 and an overlap capacitance between the gate electrode 103 and the second N type region (including the first N type region 105) 106.
As a countermeasure against such a soft error, use is made of a method for increasing an amount of signal charge by decreasing the thickness of the gate oxide film and hence increasing a storage capacity. However, decreasing the gate oxide film to an unlimited extent poses a problem against the gate oxide film. Further, the driver transistors are usually formed simultaneously with the transfer transistors and SRAM's peripheral circuits, etc., and it has been difficult to vary the thickness of the gate oxide film of the driver transistor only.
As described above, to obtain a large-capacity SRAM, it is necessary to form respective transistors in fine form. However, this leads to a decrease in an effective capacitance as the transistors are reduced. For the data-holding driver transistors, therefore, it is necessary to compensate for a decrease in the effective capacitance so as to take a soft error countermeasure. The method for achieving the thinning of the gate oxide film encounters difficulty in securing the reliability of such transistors. It is also difficult to do this from a standpoint of the technical process.
A demand has also been made for a high-speed SRAM and, therefore, it is not preferable to increase the effective capacitance for those transfer transistors for switching as well as for those transistors used in the peripheral circuits for SRAM.